In Section , we will review the construction of crossing equations, illustrating it on a simple example. The two ingredients entering the equations will be defined. These are conformal blocks and the so called crossing factors. We shall follow the influential approach of Dolan and Osborn who characterised the blocks as solutions to a set of Casimir differential equations, [1]}, [2]}. We will then comment on the extent to which both conformal blocks and crossing factors are known (they depend on spacetime dimension, amount of supersymmetry and type of fields entering the four-point function). This material is very well known and is included in order to formulate clearly the problem that we wish to address in the rest.
